A variety of artifacts are available to measure dimensional measuring devices such as coordinate measuring machines (CMM), articulated arm CMMs, laser trackers, and 3D scanners. Many types of artifacts are available including scale bars, ball bars, gage blocks, step gages, and ball plates.
One type of scale bar includes a bar onto which are mounted two or more nests, each of which can hold an object having a spherical shape. One conventional method to accurately measure the distance between the centers of spheres held in the nests includes using a calibrated interferometer within a laser tracker. A laser tracker is a device that measures the distance and two angles to a retroreflector target, which is usually a spherically mounted retroreflector (SMR). An SMR includes a cube corner retroreflector embedded within a steel sphere with the vertex of the cube corner at the center of the sphere. By lining up the laser tracker with the positions of the SMRs on the scale bar, a very accurate measurement based on tracker interferometer readings can be obtained.
To measure the distance between the centers of the nests on the scale bar, the operator aligns the laser beam with the SMR when mounted on each of the two nests. The operator reads the distance to the SMR at the first nest and then moves the SMR to the second nest without allowing the laser beam to break, which would result in count loss and invalidate the measurement. The operator then reads the distance to the second nest. The difference between the measured distances is the distance between the two nest centers. To make this SMR movement possible, the scale bar is designed to permit the laser beam from the tracker to travel between nest centers without encountering any obstructions.
Most scale bars are designed to sit on kinematic mounts. Kinematic mounts enable the scale bar to be removed and then be reseated in the same position. Conventionally, there are many kinematic mounting methods. One method brings a sphere on a first mount in contact with a plane, a sphere on a second mount in contact with a V-block, and a sphere on a third mount in contact with three smaller spheres located 120 degrees apart. The scale bar can be mounted on a flat surface such as a table. The three larger spheres can be mounted on the bottom of the scale bar and the three complementary surfaces (plane, V-block, 3-sphere structure) can be placed on the table in contact with the larger spheres. A first alternative is to mount the larger spheres on the table and the three complementary surfaces on the bottom of the scale bar. A second alternative is to mount some of the larger spheres on the table and others on the bottom of the scale bar.
Regardless of selected method, the principle involved in kinematically mounting a rigid body is to fully constrain, but not over-constrain, the rigid body. A rigid body—in this case the scale bar—has six degrees of freedom that need to be eliminated to fully constrain without over-constraining. The mount with the three small spheres constrains the larger sphere so that translational motion is not possible at the position of the larger sphere. In other words, three of the six degrees of freedom of the scale bar have been removed. The mount with the V-block constrains the second sphere to move along a straight line, thereby removing two more degrees of freedom. The mount with the flat surface constrains the third sphere to move on a plane, thereby removing one degree of freedom. By using the three mounts together, six degrees of freedom are removed, and the scale bar is fully constrained, but not over-constrained.
Another kinematic mounting method uses three pairs of cylinder, each pair spaced 120 degrees away from the other pairs and each pair pointed toward a central point. Each of the three pairs of cylinders is brought in contact with a sphere. As each cylinder constrains the sphere to move along a straight line, each removes two degrees of freedom, so that the three mounts, when placed between the scale bar and table, fully constrain motion of the scale bar.
Usually the three mounts are placed opposite the side of the scale bar that holds the magnetic nests as described herein. For example, if the mounts are placed on the bottom of the scale bar, the nests are placed on the top of the scale bar. This positioning of the mounts on the side opposite the nests prevents the laser beam from being broken as the SMR is moved from one nest to the other.
However, this placement of the mounts on the side of the scale bar opposite the nests may increase errors in the use of the scale bar. One such error occurs when forces applied to the bar causes the bar to bend. Such forces may be applied by the operator when the operator touches the scale bar with a probe of the instrument under test (for example, the probe on an articulated arm coordinate measuring machine (CMM)). Alternatively, such forces may come from gravity as the scale bar is moved to different orientations, or they may come from thermal expansion of the base element to which the scale bar mounts are attached.
An additional type of error can occur for the case of a scale bar that includes two scale bar segments mounted on a supporting structure. In this case, forces on the structure can cause movement of one of the segments, thereby resulting in a change in the length between the centers of the nests.
There is a need today for an artifact that (1) resists changes in length as forces are applied to it and (2) is free of obstructions, thereby permitting measurement of the distances between nests centers with a laser tracker interferometer.